Sheathed thermocouple

ABSTRACT

An hermetically sealed metal sheathed thermocouple insulated with magnesia or beryllia wherein air or other oxidising medium is removed from the sheath before sealing.

United States Darling atet [191 1 SHEATHED THERMOCOUPLE [75] Inventor: Alan Sydney Darling, Northwood,

England [73] Assignee: Johnson Matthey & C0,, Limited,

London, England [22] Filed: Oct. 13, 1972 [21] Appl. No.: 297,447

Related U.S. Application-Data [63] Continuation of Ser. No. 30,870, April 22, 1970,

abandoned. 1

[30] Foreign Application Priority Data 11] 3,821,030 1 June 28, v1974 FOREIGN PATENTS OR APPLICATIONS 854,570 11/1960 Great Britain.... 136/233 947,710 1/1964 Great Britain 136/236 1,351,423 12/1963 France 136/236 1,090,137 11/1967 Great Britain 136/236 OTHER PUBLICATIONS High Temperature, USSR, Vol. 2, July-August, 1964,

Primary Examinerl-Iarvey E. Behrend Attorney, Agent, or FirmCushman Darby & Cushman 571 ABSTRACT An hermetically sealed metal sheathed thermocouple insulated with magnesia or beryllia wherein air, or other oxidising medium is removed from the sheath before sealing.

6 Claims, 4 Drawing Figures 1 1450 FURNACE TEMPERATURE PROFILE 1 RHomuM CONTENT 12 OF Posmva LAMB 4500 I T 9 10 2 2 RHODIUM co m D NTENT 425mg 5 SHEATH O '2 a -1ZO0 M t 6- 4150 5 2 RHODIUM CONTENT OF NEGATIVE LIMB so 1 I Z l 4 e 8 DlSTANCE FROM HOT J'UNCTION (memes) 1 SI-EATHED THERMOCOUPLE This is a continuation, of application Ser. No. 30,870, filed Apr. 22, 1970, now abandoned.

This invention relates to a metal sheathed thermocouple having high stability at high temperatures Industrial temperature measurement in the range 1,000C 1,500C is largely dependent upon the use of rhodium-platinum thermocouples and hitherto it has not been possible to provide an alternative measuring device of comparable reliability and precision. The factors which affect the stability of noble metal couples have been carefully studied and it has now been established that when external contamination is avoided, a slow migration of rhodium from the positive to the negative limb accounts for the gradual deterioration in thermoelectric behavior which occurs at high temperatures.

Rhodium is transferred by a vapour phase reaction involving the volatile oxide of rhodium. The standard type of noble metal thermocouple has a fairly loosely fitting refractory sheath and the natural convection and ventilation which occurs in air at high temperatures generally prevents the accumulation of serious concentrations of rhodium oxide within the assembly. Different conditions exist within metal sheathed thermocouple assemblies, however, which are hermetically sealed, so that mixed platinum/rhodium oxide vapours can build up to pressures which are commensurate with oxygen partial pressure within the sheath. From these saturated oxide vapours the pure platinum limb can take up rhodium, and the view has sometimes been expressed that metal clad thermocouples, although robust and resistant to mechanical damage, must inevitably be less stable than those which are well ventilated.

It is an object of this invention to provide a sheathed thermocouple wherein the possibility of migration of rhodium through the intermediary of the oxide or other vapour phase is considerably reduced or practically eliminated.

According to one feature of the invention in a metal sheathed thermocouple insulated with magnesia or berryllia, air or other oxidising medium is removed from the interior of the sheath prior to the hermetic sealing thereof. Preferably the interior of the sheath is filled with an inert gas such as argon.

Such an arrangement reduces or eliminates oxide vapour pressures and thus rhodium migration via the oxide phase is virtually eliminated.

According to a further feature of this invention -a sheathed thermocouple having its limbs formed of noble metals or noble metal alloys is provided within the sheath with a getter such as titanium, zirconium, tantalum or other reactive metal of low vapour pressure which maintains the oxidising potential of the atmosphere within the sheath at a very low level.

When extreme stability is required, however, vapour pressures of the metals themselves must also be considered.

It is another object of this invention to provide a construction of sheathed thermocouples wherein any harmful effects caused by rhodium or other. metal vapour migration is reduced to negligible levels by the suitable selection of the constituents of which the thermocouple and its sheath are constructed.

According to another feature of the invention in a sheathed thermocouple having its limbs formed of noble metals or noble metal alloys the sheath contains a smaller percentage of the more volatile constituent of said limbs than is present in either of said limbs, or none of said constituent.

The invention thus includes a thermocouple having an hermetically sealed metal sheath and provided with means to maintain the oxidising potential of the atmosphere within the sheath at a very low value and with means to reduce the transfer of the more volatile metal constituent of the limbs to the negative limb.

It has been found that very satisfactory results are obtained when a 30 percent rhodium-platinum versus 6 percent rhodium-platinum thermocouple is insulated with magnesia and sealed within a 5 percent rhodiumplatinum sheath from which air has been removed. With this arrangement the vapour pressure of rhodium in equilibrium with the sheath alloy is, at any temperature, lower than that which is in equilibrium with either of the thermocouple wires. Any rhodium vapour generated by the couple wire is immediately dissolved by the sheath, which, because of its considerably greater volume, is capable of acting as arhodium getter for prolonged periods. The net effect of rhodium migration is a slow and gradual fall in the rhodium content of the 30 percent rhodium-platinum wire, which, however, has little effect upon the thermoelectric power of the couple.

A sheathed thermocouple of the above construction has recently been tested at l-,450C. Tests were made in air, and the indicated temperatures were compared to those given by a 20/40 rhodium-platinum thermocouple of simple bare wire construction which was inserted into the furnace about once everyv 24 hours. The results obtained over a 30 day period of test showed no steady drift in calibration and only slight random variations (in general less than 1C) between the temperature indications of the two thermocouples. Slight grain growth of the sheath alloy near the hot junction was the only visible evidence of deterioration.

The invention will be hereinafter more fully described by way of example with reference to the accompanying diagrammatic drawings in which:

FIG. 1- is a graph showing a comparison between known thenno-couples and one constructed according to the invention;

FIGS. 2 and 3 indicate graphically changes in the content of thermocouple limbs and sheath in a known thermocouple and in one according to'the invention re spectively, during operation;

FIG. 4 shows graphically the influence on the performance of varying the constituents of the sheath.

Referring to FIG. 1, the thermo-electric performance and stability of several rhodium-platinum thermocouples are compared over test periods extending up to 1,500 hours. Curves A, B, C, D and E show the performance in air of five 13 percent rhodium-platinum versus platinum thermocouples selected at random from existing stocks, the thermocouples being clad in 10 percent rhodium-platinum sheaths. Curve G shows the performance of a thermocouple accordingto the invention constructed of a negative limb of 6 percent rhodium-platinum and a positive limb of 30 percent rhodium-platinum enclosed in a sheath of 5 percent rhodium-platinum.

As will be clear, the latter thermocouple remained stable for approximately 550 hours at 1,450C and then started to become slightly unstable though the rate of fall in output was very slow and after 1,500 hours at l,450C the output of this thermocouple had fallen only by 19C. On the other hand the other five thermocouples whose performances are indicated by the curves A E exhibit fairly'rapid decline in performance which in some instances was as high as 6C per day.

It has been found that the long-term stability of the 6 percent rhodium-platinum versus 30 percent rhodium-platinum thermocouple sheathed in percent rhodium-platinum is higher than the most stable of the known 13 percent Rh-Pt versus Pt thermocouples sheathed in 10 percent Rh-Pt.

The above described stability tests were carried out in a 35 inch long resistance tube furnace having a 2 inch bore and heated by 10 percent Rh-Pt elements. The tests were made in air at 1,450C and. the tips of the metal clad thermocouples were inserted in a 10 percent Rh-Pt block drilled to receive the couples and centrally disposed in the tube furnace at the middle of the uniformly heated length.

Two recrystallised alumina sheaths 18 inches long and having a bore of 5 mm were inserted in the Rh-Pt block similarly to the thermocouples, one of which sheaths contained a twin bore alumina insulated platinum versus 13 percent rhodium-platinum thermocouple acting as a permanent furnace temperature indicator. The other sheath was intended to receive occasionally a standard 13 percent Rh-Pt versus Pt thermocouple against which the output of the test samples could be compared.

The reason for the superiority of the 6 percent Rh-Pt versus 30 percent Rh-Pt clad in a 5 percent Rh-Pt sheath according to this invention will be understood from the analytic data set out in FIGS. 2 and 3, showing the composition of the two limbs of each thermocouple combination after a'prolonged test. This composition is indicated as the rhodium content plotted as a function of the distance from the hot junction of the thermocouple under test. A profile of the furnace temperature gradient over the same test length is superimposed on each diagram.

FIG. 2 refers to the 13 percent Rh-Pt versus Pt thermocouple analysed after 400 hours at 1,450C. The pure platinum limb has taken up 3.8 percent by weight of rhodium at the hot junction, the rhodium content of the positive limb has decreased from 13 percent to 9.6 percent and the content of rhodium in the sheath has changed from 10 percent to 8.2 percent. These changes in composition explain the deterioration in thermoelectric performance and due to the relative volume of the sheath as compared with those of the limbs it will be clear that the sheath itself acts as the'main reservoir of rhodium which contaminates the platinum limb.

On the other hand the conditions in the thermocouple sheathed in 5 percent Rh-Pt according to the invention are much more stable as shown in FIG. 3. In this case the composition of the 6 percent Rh-Pt limb has changed very slowly throughout the test which lasted for 2,200 hours. The sheath had only very slightly changed in composition but the rhodium content of the positive limb of the thermocouple at the cold junction had decreased from 30 percent to 20 percent. In spite of this change in composition the thermo-electric output of the thermocouple had only slightly changed mainly due to the low rate of change of E. M. F. with change in composition of alloys containing substantial quantities of rhodium.

In this form the 5 percent Rh-Pt sheath had not contributed any rhodium to the negative limb of the thermocouple, although however, it must have extracted considerable rhodium from the positive limb. The slight decrease in rhodium content of the sheath may be explained by continuous removal of rhodium oxide from its outer surface by the scrubbing action of the air.

FIG. 4 shows the results of further stability tests made with 6 percent Rh-Pt versus 30 percent Rh-Pt thermocouples clad in various sheath materials. The deviations in C are plotted against time and the above thermocouple clad in a sheath of 5 percent Rh-Pt alloy is shown in Curve L which follows the general line of Curve G in FIG. 1. When a sheath of 20 percent Rh-Pt alloy was substituted for 5 percent Rh alloy the results followed Curve M from which it is clear that deterioration occurs when the sheath contains more rhodium than the negative limb of the thermocouple. Curve N shows a thermocouple of 20 percent Rh-Pt versus 40 percent Rh-Pt in a sheath of 30 percent Rh-Pt alloy which shows a greater deterioration than Curve M.

Theoretically the rhodium-platinum alloy of the sheath can be reduced in rhodium content down to pure platinum. It was considered, however, that the 5 percent rhodium alloy being close to the composition of the negative limb would help to stabilise the behaviour of the negative limb and the alloy addition provided an important contribution to the strength of the sheath. A pure platinum sheath would be weak mechanically and would have had such ahigh affinity for rhodium that it might well have depleted the 6 percent rhodium-platinum limb.

The invention is obviously applicable to noble metal thermocouples incorporating other noble metal alloys. For example a high temperature thermocouple operative within the'temperature range l,750 2,lOOC is known having its limbs formed of iridium and 40 percent lr/Rh respectively. In such a case a sheath of pure rhodium would be used. In this, as in the previous example, air and/or oxygen would have to be removed from the interior of the sheath because of the readiness with which iridium oxidises.

While pure platinum as abovementioned forms a mechanically weak sheath, satisfactory sheaths may be formed of dispersion strengthened platinum. This will permit the sheath to act as an absorberof rhodium, iridium, ruthenium and molybdenum while retaining sufficient strength for practical use.

For example, thermocouples may be formed wherein the positive limb contains 10 13 percent of rhodium, the negative limb may contain l percent of rhodium or may be of pure platinum, while the sheath is of pure platinum.

'Altematively, with the negative limb and the sheath being of pure platinum, the positive limb may consist of a 10 percent iridium-platinum alloy. Further, the limbs may be formed of dilute molybdenum platinum alloys or of dilute ruthenium platinum alloys with the sheath being of pure platinum.

What we claim is:

l. A metal sheathed thermocouple comprising a positive rhodium/platinum alloy limb and a negative rhodium/platinum alloylimb wherein undesired migration of rhodium metal vapor from one limb to the other is reduced, said limbs being positioned within a hermetically sealed rhodium/platinum alloy sheath, the atmosphere within said sheath being inert, the alloys of the positive and negative limbs and the alloy of the sheath each consisting essentially of platinum with a lesser amount of rhodium, the alloys of said limbs containing different quantities of rhodium and the alloy of the sheath containing a quantity of rhodium which is less than the quantity of rhodium in the limb containing the lesser quantity of rhodium whereby in use the vapour pressure of rhodium metal vapour in equilibrium with the sheath is lower than the vapour pressure of that which is in equilibrium with either of said limbs so that any rhodium metal vapour generated by said limbs is preferentially dissolved by said sheath.

2. A thermocouple according to claim 1 wherein the negative limb contains 6 percent rhodium, the positive limb contains 30 percent rhodium while the sheath contains 5 percent rhodium.

3. A thermocouple according to claim 1 wherein the sheath is filled with an inert gas.

4. A thermocouple according to claim 1 wherein the limbs of the thermocouple are insulated from each other and from the sheath by a member of the group consisting of magnesia and beryllia.

5. A metal sheathed thermocouple according to claim 1 wherein one limb comprises a 30 percent rhodium-platinum and the other limb comprises 6 percent rhodium-platinum, said limbs being insulated with magnesia and sealed within a 5 percent rhodium-platinum sheath from which air has been removed.

6. A metal sheathed thermocouple comprising a positive rhodium/platinum alloy limb and a negative rhodium/platinum alloy limb wherein undesired migration of rhodium metal vapour from one limb to the other is reduced, said limbs being positioned within a hermetically sealed rhodium/platinum alloy sheath, a getter within the sheath, said getter being a reactive metal selected from the group consisting of titanium, zirconium and tantalum, the alloys of the positive and negative limbs and the alloy of the sheath each consisting essentially of platinum with a lesser amount of rhodium, the alloys of said limbs containing different quantities of rhodium and the alloy of the sheath containing a quantity of rhodium which is less than the quantity of rhodium in the limb containing the lesser quantity of rhodium whereby in use the vapour pressure of rhodium metal vapour in equilibrium with the sheath is lower than the vapour pressure of that which is in equilibrium with either of said limbs so that any rhodium metal vapour generated by said limbs is preferentially dissolved by said sheath. 

2. A thermocouple according to claim 1 wherein the negative limb contains 6 percent rhodium, the positive limb contains 30 percent rhodium while the sheath contains 5 percent rhodium.
 3. A thermocouple according to claim 1 wherein the sheath is filled with an inert gas.
 4. A thermocouple according to claim 1 wherein the limbs of the thermocouple are insulated from each other and from the sheath by a member of the group consisting of magnesia and beryllia.
 5. A metal sheathed thermocouple according to claim 1 wherein one limb comprises a 30 percent rhodium-platinum and the other limb comprises 6 percent rhodium-platinum, said limbs being insulated with magnesia and sealed within a 5 percent rhodium-platinum sheath from which air has been removed.
 6. A metal sheathed thermocouple comprising a positive rhodium/platinum alloy limb and a negative rhodium/platinum alloy limb wherein undesired migration of rhodium metal vapour from one limb to the other is reduced, said limbs being positioned within a hermetically sealed rhodium/platinum alloy sheath, a getter within the sheath, said getter being a reactive metal selected from the group consisting of titanium, zirconium and tantalum, the alloys of the positive and negative limbs and the alloy of the sheath each consisting essentially of platinum with a lesser amount of rhodium, the alloys of said limbs containing different quantities of rhodium and the alloy of the sheath containing a quantity of rhodium which is less than the quantity of rhodium in the limb containing the lesser quantity of rhodium whereby in use the vapour pressure of rhodium metal vapour in equilibrium with the sheath is lower than the vapour pressure of that which is in equilibrium with either of said limbs so that any rhodium metal vapour generated by said limbs is preferentially dissolved by said sheath. 